The present embodiments relate to generation of a radio-frequency pulse for excitation of nuclear spins in a predetermined layer of a specimen for magnetic resonance imaging.
Magnetic resonance imaging is known from the prior art as an imaging method with which hydrogen density and bonding conditions in an object under examination are determined from an excitation of nuclear spins of protons in a nucleus of the hydrogen in an external magnetic field B using an external radio-frequency signal and a radio-frequency measuring signal emitted thereupon by the object under examination. The radio-frequency measuring signal is converted into a visual representation of the object under examination. During this, for example, hydrous tissue or other substances with a nuclear spin different from zero may be detected.
The quality of the radio-frequency measuring signal and the images generated therefrom increases with the strength of the applied static external magnetic field B since, with an increasing magnetic field B, the energy distance of the states of the proton in the magnetic field increases. This has the result that an occupation difference, and hence the signal strength, increases in the thermal equilibrium. Field magnets for magnetic resonance imaging have, for example, magnetic fields between 1.5 T and 3 T and up to 7 T.
The radio-frequency signal used for the excitation of the nuclear spins is irradiated at the Larmor frequency, the resonance frequency of the nuclear spins in the external magnetic field. The Larmor frequency is proportional to the magnetic field strength B, and the specific absorption rate (SAR) of the electromagnetic radiation in the body is proportional to the square of the frequency f. Overall, the following is obtained for the specific absorption rateSAR ∝B2θ2 Δfwhere θ is the flip angle with which the nuclear spin is to be tilted in alignment, and Δf is the bandwidth of the excitation pulse. The bandwidth is determined by the variation of the magnetic field B in a measuring volume for which imaging is to be performed. In addition, a safety margin is to be provided in the bandwidth in order to take account of deviations of the magnetic field from the desired field strength that may be caused, for example, by the body of the actual patient or the environment of the magnetic resonance imaging device. If the safety margin in the bandwidth is set too low, the result is that nuclear spins are not excited or are excited in offset regions. During the reconstruction of the image, this has the result that these regions are not depicted or are depicted on incorrect coordinates and hence generate artifacts in the image.
For magnetic resonance imaging devices with magnetic fields of 3 T, peak pulse powers of the excitation pulses of 35 kW are achieved. Only a small part of this power is used for the excitation of the nuclear spins, while the majority is converted into heat in the body of the person to be examined.
From a certain degree of heating, there is a risk to the health of the person to be examined. For example, the eyeball is particularly sensitive to heating and may become opacified. For this reason, there are limit values specific to each country for a maximum allowable SAR. In magnetic resonance imaging devices with 3 T or more, therefore, the power of the excitation pulses is to be reduced, or the time intervals between the pulses are to be increased in order to adhere to a permissible mean value for the SAR. The result is that the examination lasts longer, or the image quality is below the technical achievable level with respect to resolution or the signal-to-noise ratio.